Antonello Simonetti | University of South Carolina (original) (raw)

Papers by Antonello Simonetti

"Integration of new jumbo piston cores with high-resolution seismic reflection data prov... more "Integration of new jumbo piston cores with high-resolution seismic reflection data provides the first documentation on the subsurface distribution of gas hydrates at a deep-water cold seep. The study area, Woolsey Mound is a carbonate/hydrate mound of thermogenic origin, in the northern Gulf of Mexico where salt tectonics dominate the regional structure. Two typologies of unconventional seismo-acoustic data were used in this study: 1) surface-source deep-receiver (SSDR) data and 2) AUV-borne chirp subbottom profiler data. Correlation of the coring results with seismic interpretations supports the hypothesis that distinctive seismic brightening (high frequency scattering) present in the SSDR records may indicate the presence of solid hydrates. This suggests that such unconventional seismic survey may be prospective for mapping gas hydrates in complex deep-water settings. Coring results revealed the nature of the shallow gas hydrate system to be dominated by fine-grained sediments. Gas hydrates were found exclusively in fracture porosity in the vicinity of a major active fault. We present a model for Woolsey Mound where shallow gas hydrates are systematically distributed along segments of faults intersected by transit of thermogenic hydrocarbons. This dual nature of the faults being both gas hydrate reservoirs and gas migration routes suggests a very dynamic hydrate stability field for this site."

Three Upper Miocene hardgrounds have been analysed in this study, outcropping in the Latium-Abruz... more Three Upper Miocene hardgrounds have been analysed in this study, outcropping in the Latium-Abruzzi Apennines (Italy). The central Apennine hardgrounds all lie on top of the Latium-Abruzzi carbonate ramp succession and in each case are overlain by hemipelagic Orbulinamarls; these marls are linked to plate flexure-related to drowning and coeval input of terrigenous sediments. The hardground age ranges from Tortonian to Early Messinian. Phosphate precipitation in the investigated hardgrounds was confined to a thin layer (up to 15 cm) close to the sediment water interface. Here oxic to suboxic conditions prevailed, resulting in early-diagenetic iron cycling and subsequent phosphogenesis in oxygenated bottom-waters. Glaucony only occurs in the planktonic-rich marls that overlie and infill the phosphatized hardground level in the Latium-Abruzzi succession. An upwelling flux triggered phosphogenesis, promoting the early lithification of the sea floor on the platforms. After upwelling event neritic carbonate production could not be re-established on the Latium-Abruzzi platform because of the persisting eutrophic conditions and the high rates of tectonic subsidence and terrigenous input linked to Appennine orogenesis. The Latium-Abruzzi phosphorites are coeval with the Tortonian phosphogenic phase reported in the Mediterranean. Despite being a global event, regional and local factors played a major role in the hardground deposition at each site.

Geochemical profiles were coupled with seismic information to examine subsurface hydrocarbon sour... more Geochemical profiles were coupled with seismic information to examine subsurface hydrocarbon source, migration, and fate at a Gulf of Mexico carbonate-gas hydrate mound (Woolsey Mound). Three seafloor features were investigated in detail: 1) major faults resulting from a rising salt body, 2) an acoustic backscatter anomaly, and 3) a pockmark associated with a major fault. We analyzed sulfate, chloride, dissolved inorganic carbon, and hydrocarbon concentrations, and carbon isotopes in porewater extracted from 20 m piston cores to characterize gas source and calculate methane flux. Dissolved biogenic methane dominated the off-fault sites, while the contribution of thermogenic methane increased near a major fault where thermogenic gas hydrates were recovered. Within the pockmark, methane concentrations were low and isotopes indicated a biogenic source. Since pockmarks are typically formed from expulsive fluid flow, this suggests that either the pockmark is the legacy of a conduit that has become plugged or that the expulsed fluid is confined within the fault walls. At the acoustic anomaly, non-steady state sulfate profiles suggested temporal variability in methane flux. Estimates from >75 gravity cores collected across Woolsey Mound since 2002 were mapped to display the spatial variability in methane flux relative to the faults. Methane flux to the seafloor was generally low, but increased several fold near the faults suggesting that the faults may provide conduits for hydrocarbons to bypass the 'microbial biofilter' and cross the sediment water interface.

"Integration of new jumbo piston cores with high-resolution seismic reflection data provides the ... more "Integration of new jumbo piston cores with high-resolution seismic reflection data provides the first documentation on the subsurface distribution of gas hydrates at a deep-water cold seep. The study area, Woolsey Mound is a carbonate/hydrate mound of thermogenic origin, in the northern Gulf of Mexico where salt tectonics dominate the regional structure. Two typologies of unconventional seismo-acoustic data were used in this study: 1) surface-source deep-receiver (SSDR) data and 2) AUV-borne chirp subbottom profiler data. Correlation of the coring results with seismic interpretations supports the hypothesis that distinctive seismic brightening (high frequency scattering) present in the SSDR records may indicate the presence of solid hydrates. This suggests that such unconventional seismic survey may be prospective for mapping gas hydrates in complex deep-water settings. Coring results revealed the nature of the shallow gas hydrate system to be dominated by fine-grained sediments. Gas hydrates were found exclusively in fracture porosity in the vicinity of a major active fault. We present a model for Woolsey Mound where shallow gas hydrates are systematically distributed along segments of faults intersected by transit of thermogenic hydrocarbons. This dual nature of the faults being both gas hydrate reservoirs and gas migration routes suggests a very dynamic hydrate stability field for this site."

The northern Gulf of Mexico is dominated by salt tectonics, resulting fracturing and numerous sea... more The northern Gulf of Mexico is dominated by salt tectonics, resulting fracturing and numerous seafloor seeps and vents. Woolsey Mound, site of the Gulf of Mexico Hydrates Research Consortium's seafloor observatory, has been investigated extensively via surveys, direct sampling and seafloor instrument systems. This study presents an innovative approach to seismic data interpretation, integrating three different resolution datasets and maximizing seismic coverage of the complex natural hydrocarbon plumbing system at Woolsey Mound.

3D industry seismic data reveal the presence of a salt body at in the shallow subsurface that has generated an extended network of faults, some extending from the salt body to the seafloor (master faults). Higher resolution seismic data show acoustic wipe-out zones along the master faults with expulsion features – seafloor pockmarks and craters – located immediately above them and associated, in the subsurface, with high-amplitude, negative anomalies at constant depth of 0.2 s TWTT b.s.f., interpreted as free gas. Since pockmarks and craters provide pathways for hydrocarbons to escape from depth into the water column, related sub-surface seismic anomalies may indicate free gas at the base of the gas hydrates stability zone (GHSZ). Fluid flow and gas hydrates formation are segmented laterally along faults. Gas hydrates formation and dissociation vary temporally in the vicinity of active faults, and can temporarily seal them as conduits for thermogenic fluids. Periodic migrations of gases and other fluids may perturb the GHSZ in terms of temperature and pressure, producing the observed lack of classical BSRs.

Located on the continental slope in 900m of water, Woolsey Mound dominates seafloor morphology at... more Located on the continental slope in 900m of water, Woolsey Mound dominates seafloor morphology at Mississippi Canyon 118. The carbonate-hydrate mound is the site of the Gulf of Mexico Hydrates Research Consortium’s seafloor observatory to investigate and monitor hydrographic, geophysical, geological, geochemical and biological processes of the hydrocarbon system, northern Gulf of Mexico. Spatial and temporal variability of processes that produced the mound - venting fluids, formation/dissociation of hydrates, formation of authigenic carbonate and of micro and macrofaunal communities - are unknown and form the basis of several investigations at the site.

Innovative survey and monitoring systems, sensors, and tools have been developed to extract samples and data to unravel the history, character and composition of the site. This study represents an attempt to integrate results of extensive geophysical studies with recent studies of the macrofauna thriving at the site, and to use the results to develop a system of vent classification for use in evaluating the subseafloor hydrocarbon system.

Seafloor morphology and geology have been characterized integrating high resolution swath bathymetry, acoustic imagery, seafloor video and sediment, water column, and pore-water samples. AUV bathymetric surveys were completed in 2005 and 2009; video images, photographs, core samples and water samples were collected during cruises from 2002 - 2010.
Seismo-acoustic data have been directed at maximizing definition at various depths. Deep data show a salt body underlying Woolsey Mound; crestal faults emanating from the salt body infrequently but notably intersect the seafloor. High frequency chirp and surface-source-deep-receiver data reveal many intersections of antithetical faults with the seafloor. These have been mapped over seafloor bathymetry determined from multibeam surveys.

Outcropping hydrates, fluid-migration features and seafloor communities- identified and described from numerous types of imagery - have been mapped. These maps have been combined/overlain on the bathymetry/fault maps to produce a biotypes-seep map from which we have identified and differentiated types of seeps. Community complexity is used as a proxy for seep duration/age while specific community components are believed to reflect composition of seep fluids. Although preliminary, this approach represents a novel classification system for seafloor hydrocarbon seeps.

Conference Presentations by Antonello Simonetti

We investigated Woolsey Mound, a hydrate/carbonate mound associated with a fault-controlled cold ... more We investigated Woolsey Mound, a hydrate/carbonate mound associated with a fault-controlled cold seep in the continental slope of the Northern Gulf of Mexico. Here, thermogenic gas hydrate accumulations along active faults nucleating from the underlying salt dome, and intermittent seafloor hydrocarbon vents coexist. The aim of our study was then to understand the nature and the time scale of the processes governing gas hydrate destabilization and subsequent hydrocarbon venting into the water column. Results from time-lapse seismic monitoring conducted using two sets of 3-D standard seismic data acquired three years apart, suggest that the upward migration of deep-sourced hydrocarbons and brines along the faults may be the primary short-term control in the destabilization of gas hydrates. Such a hypothesis would be plausible considering that the fault network not only constitutes the reservoir for gas hydrates but it also provides the main migration pathways for arising thermogenic hydrocarbons. Additionally, two subsurface geophysical anomalies, which may be buried paleo-mounds, have been identified on high resolution 2-D seismic data at approximate depths of 50 m and 150 m b.s.f. While the stratigraphy separating the two paleo-mounds is uniform, they appear to be uniquely correlated with two localized growth sequences along major faults, indicating a syn-tectonic stage during their formation. Such observations suggest that the two paleo-surfaces developed in a setting analogous to the present-day tectonically active scenario at Woolsey Mound, where the transit of hydrocarbons along the faults repeatedly triggered gas hydrates destabilization and hydrocarbons venting at the seafloor. Hence, we speculate that the long-term (tens to hundreds of thousands of years) mechanisms controlling the stability of the gas hydrate-cold seep system at Woolsey Mound may be driven by tectonic activity and quiescence.

Gas hydrate (GH) bearing sediments in oceans are thought to be the Earth’s largest reservoir of m... more Gas hydrate (GH) bearing sediments in oceans are thought to be the Earth’s largest reservoir of methane (CH4), accounting for about 7.4 × 10^15 tonnes of CH4 at STP (Milkov, 2003; Buffet and Archer, 2004). Thus, in marine settings, particularly in cold seep environments, venting from GH deposits introduces significant volumes of CH4 from the lithosphere into the hydrosphere and, to some extent, the atmosphere (Leifer and Boles, 2005). Should ocean temperatures continue to rise, a substantial amount of CH4 could potentially be released from GH reservoirs. However, the fate of ocean floor GH not only depends on fluctuating temperatures: pressure, salinity, hydrocarbon availability and composition, and the presence of certain catalysts and/or inhibitors, are key factors controlling the gas hydrate stability field (GHSF). Consequently, numerous physical, chemical, geological and biological processes, at different time scales, promote GH formation, accumulation, dissociation and subsequent venting of CH4 into the water column. Given methane’s relevance as a greenhouse gas, its potential impact on ocean acidification and supporting cold seep communities (Bangs et al., 2011), the understanding of the dynamics involved in GH systems becomes therefore crucial.
This study shows the results of a time-lapse seismic monitoring (4D seismic) conducted at Woolsey Mound (MC118), a fault-controlled cold seep located at 900m water depth in the continental slope of the northern Gulf of Mexico. Here, thermogenic GH appear to be temporarily stable both at the seafloor and in the shallow subsurface, and associated chemosynthetic communities exhibit a different degree of evolution, suggesting high fluid-flux variability through time within the GH system. Quantitative analysis through amplitude differencing of two sets of 3D seismic data acquired 3 years apart, reveals short-term changes in subsurface anomalies. We interpret these to be the result of actively migrating hydrocarbon fluids and the mechanisms that trigger GH dissociation appear to be primarily controlled by transient fluid-dynamic processes along the major faults previously identified as hydrocarbons migration pathways. However, the causes of these short-term changes are still contentious. We speculate that transitory shallow GH formation along the faults obstructs thermogenic hydrocarbons migrating upwards, until a pressure threshold is reached and hydraulic fracturing promotes GH dissociation and subsequent venting into the water column. Yet, seasonal variations in ocean bottom current temperatures and complex bio-geochemical interactions may play an important role in destabilizing shallow GH reservoirs.

Antonello Simonetti | University of South Carolina (original) (raw)

Papers by Antonello Simonetti

Conference Presentations by Antonello Simonetti

Talks by Antonello Simonetti